There is a security feature in the VPN client that blocks local connections, including local Internet access, to protect the office/remote network. You can disable this if you wish. To do so on the client/connecting PC, go to:
control panel | network connections | right click on the VPN/Virtual adapter and choose properties | Networking | TCP/IP -properties | Advanced | General | un-check "Use default gateway on remote network"

If still having troubles, make sure the PPP/VPN adapter is at the top of the binding order list [ control panel | network connections | on the menu bar advanced | advanced settings | adapters and bindings ]

A totaly different approach would be to configure your Exchange server and clients for rpc/http e-mail. This is more secure and requires no VPN. If using pre-Exchange 2003, I don't belive this is an option.

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At a command line try using trace route to a web site such as Google:
tracert 64.233.187.99
You shouldn't see any IP's from the corporate office present. If not familiar with tracert, it takes a few minutes to run, give it time.